Cholangiocytes are the target cells in cholestatic liver diseases such as primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC), which are characterized by the damage and proliferation of these cells. Cholangiocyte proliferation is critical for the maintenance of the biliary mass and secretory function during the pathogenesis of these devastating liver diseases. Proliferating cholangiocytes serve as a neuroendocrine compartment during the progression of liver diseases, and as such secrete a number of growth factors, and respond to hormones, neurotransmitters and neuropeptides contributing to the autocrine and paracrine pathways that modulate liver inflammation and fibrosis. The overall objective of this proposal is to determine the molecular mechanisms by which local (from cholangiocytes), paracrine (by administration of melatonin or anti-melatonin antibody) and central (from pineal gland) melatonin synthesis (regulated by the enzyme AANAT expressed in the liver mainly by cholangiocytes) participates in the homeostasis of the biliary mass during cholestasis and liver damage. We proposed the central hypothesis that local melatonin synthesis in cholangiocytes as well as central secretion of melatonin from the pineal gland (pineal gland/biliary axis) coordinately regulates biliary proliferation/damage during cholestasis. To test the central hypothesis, we have proposed two specific aims: (1) Evaluate the effects of the modulation of melatonin synthesis by pineal gland during cholestasis on cholangiocyte proliferation; and (2) Define the role of local melatonin synthesis in the regulation of the proliferation/damage of cholangiocytes to cholestasis. At the completion of this project, we expect to have determined the extent to which proliferation of cholangiocytes is regulated by central and local melatonin synthesis, and to have identified key signaling pathways by which melatonin regulates biliary proliferation/damage. The information gained from the successful completion of these studies are expected to provide important insights into the intracellular mechanisms regulating cholangiocyte proliferation, which will ultimately help in the identification of specific factors and pathways that can be targeted for the development of therapeutic interventions. Finally, results of these studies may impact the development of improved therapeutic strategies (targeting AANAT) for the treatment of chronic cholestatic liver diseases.